The present invention relates to a semiconductor device using an insulation coated metal substrate in which semiconductor elements are supported on the insulation coated metal substrate and the semiconductor elements are connected to a wiring made of a metal foil on the substrate.
Lately, remarkable development of electronic equipment has appeared and higher density integration and light-weight miniaturization are rapidly promoted. As a leading semiconductor device in the development, power modules which are widely used in inverters and others of motor controls and air conditioners are known.
FIG. 6 shows a common three-phase inverter circuit used for motor control. In FIG. 6, IGBTs are main circuit switching elements. In this example, six IGBTs are connected between a dc main electric power V by three-phase bridge coupling. A motor 9 is driven by a three-phase ac output of the inverter. A high speed diode 8 for transferring a current is connected to each IGBT 7 in reverse-parallel. A drive circuit 6 for driving IGBT 7 is provided for each IGBT. A drive electric power 10 is a dc electric power for the drive circuit 6. Such a circuit is actually assembled as shown in FIG. 7.
FIG. 7 shows a conventional package structure for a semiconductor device. In FIG. 7, a metal base plate 13 for heat radiation, a resin case 15 fixed by an adhesive so as to surround the base plate 13, and a case cover 16 for closing an upper opening of the case 15 constitute a container of the package, in which an assembly of module circuits comprised of semiconductor elements 12 mounted on insulation coated metal substrates 11 is incorporated and further a sealing resin 4 for sealing the interior of the container is filled. An external lead terminal 14 is extracted from the circuit assembly.
The power module is made up with a plurality of semiconductor elements which are supported on a substrate and connected with the wirings provided on the substrate. Previously, an insulated substrate made of a ceramic material such as Al.sub.2 O.sub.3 has been used and a relatively thick copper base has been soldered to the substrate for the purpose of thermal radiation and the semiconductor elements have been fixed onto the copper base. However, to ensure higher density integration and light-weight miniaturization, an insulation coated metal substrate has been often used in which an aluminum plate with an excellent heat radiation efficiency is used as the substrate, an insulation layer which comprises an organic insulation is formed on this aluminum plate, copper foil of several tens of .mu.m in thickness is deposited on the insulation layer and the wiring is formed according to an appropriate patterning.
In case of the insulation coated metal substrate of such power module, an organic material which forms the insulation layer is apt to deteriorate due to corona discharge when a high voltage is applied irrespective of mounted elements and circuit structure and, if the voltage is continuously applied, dielectric breakdown will be finally caused. It is known from the measurement that the corona discharge starting voltage of the substrate alone is 400 to 600V. And it is known from the investigation that the portion where the corona was generated is an edge part of the wiring pattern which comprises copper foil (see a closed dot line E in FIG. 2 or 3). It appears that this is because the field concentrates to be increased at the edge of the copper pattern. For example, when a voltage of 200 V is applied to a metal substrate with an insulation layer of 100 .mu.m thickness, a large electric field of 2 kV/mm is applied to the substrate and the electric field concentrates to the edge of the copper pattern. As a measure, it can be considered that, for example as shown in FIG. 2, the edge part A of the pattern of Cu foil 3 formed on an aluminum substrate 1 via an insulation layer 2 made of an organic material is covered with a sealing material 4 such as molded resin so that the edge part may not be exposed to air and the corona discharge starting voltage may be increased. However, a preventive effect of this sealing material against deterioration of insulation was insufficient.